The invention relates to an object detection system and in particular, but not exclusively, to a system for detecting the presence of a metallic cooking utensil on a non-metallic cooking surface. Such systems provide enhanced safety for cooking surfaces since heating is provided only when a cooking pot is in place on the cooking surface.
Cooking platforms capable of pot detection are well known in the art, and operate according to a variety of principles. Capacitive systems are known from, for example, EP-A-0 429 120 (U.S. Pat. No. 5,136,277), DE-A-42 24 93934, DE-A-28 31 858, DE-A-37 33 108 and DE-A-38 43 460. Optical detection systems are known from DE-A-35 33 997 and DE-A-31 17 205 and acoustic systems are known from DE-A-36 19 762. A system in which reflected radiation is detected is known from DE-A-197 29 418. Other systems include active components in the cooking pots which interact with transmitters and receivers on the cooker.
A further type of known pot detection system is one where the inductive properties of metallic cooking utensils are used to modify a magnetic field generated in the vicinity of a cooking element and hence enable the detection of the pot. A first group of inductive based systems detect a change in resonant frequency of a circuit attached to a sensor coil placed in the vicinity of a cooking element. Examples of such systems are disclosed in EP-A-0 469 189 and EP-A-442 275 (U.S. Pat. No. 5,296,684).
A second group of inductive detection systems comprise a magnetic field source in the region of a cooking element and a sensor inductively coupled thereto. Placing a metallic object in the vicinity of the source influences the inductive coupling to the sensor in a manner which can be detected. An example of such a system is described in DE-A-37 11 589. In the system described therein an a.c.-operated magnetic field generator, placed at a distance below a cooking area, generates a magnetic field directed towards the cooking area. A loop lying in the external boundary area of the a.c. field is used to monitor the influence on the a.c. field of a container placed on the cooker and thereby control the switching on and off of the heating element.
A further inductive system of the second type is described in DE-A-197 00 753 a double loop arrangement is employed in which a driver loop is attached or deposited on the underside of a glass-ceramic cooking surface. This driver loop is used to generate an RF magnetic field. One or more sensor loops are arranged within or around the driver loop and these are used to generate a voltage signal which is dependent on the magnitude of the time varying magnetic field therein. If a metallic or metal containing cooking pot is placed over the driver loop, eddy currents are induced therein which have the effect of reducing the net magnetic flux in the sensor loops. Placing a pot on the driver loop therefore has the effect of reducing the voltage generated by the sensor loop. Information regarding the presence of a cooking pot can therefore be derived from this induction signal.
In the system described in DE-A-197 00 753 transformers are incorporated between a current source and the drive loop and also between the sensor loops and the detecting electronics. It is presumed by the present inventor that such transformers are provided in order to provide isolation from electrostatic charges. Since a conventional cooking surface comprises four or five cooking areas, the number of transformers required would add significantly to the cost of manufacture of such an arrangement. Furthermore, it would appear that a separate detection circuit is provided for each sensor loop.